31 No. 1
Periodic Tables and IUPAC
by G. Jeffery Leigh
|Monument honoring Dmitri Mendeleev and the periodic table, in front of the Faculty of Chemical and Food Technology of the Slovak University of Technology in Bratislava, Slovakia.
In more than one IUPAC publication you will find a periodic table, sometimes referred to as an IUPAC Periodic Table. Invariably this version of the table is what was once known as the long form, in which the various Groups are numbered from 1 to 18 (figure 1). The so-called short form cannot display the 1-18 numbering, and has largely fallen into disuse. The implication is that this is an IUPAC-approved table (see, for example, Lavelle, 2008), and indeed a Google search of the Web will unearth several entries for “IUPAC approved Periodic Table.” Figure 1 is based upon the periodic table that appears on the IUPAC website, though nowhere is it stated that that version is “approved.” In fact, IUPAC has not approved any specific form of the periodic table, and an IUPAC-approved form does not exist, though even members of IUPAC themselves have published diagrams titled “IUPAC Periodic Table of the Elements.” However, the only specific recommendation IUPAC has made concerning the periodic table covers the Group numbering of 1–18.
The usual presentation of the periodic table which we use (figure 1) is subject to continual updating as new elements are produced (Holden and Coplen, 2004). Nevertheless the conventional long form with the 1–18 group numbering, which goes back at least to the 1920s (Paneth, 1923), is certainly not ideal for every purpose. Teachers especially find that it does not fulfill all their requirements. For example, it is not strictly consistent with all our ideas about electronic structure.
The periodic table was developed from considerations of chemical properties and atomic weights and even then the latter arrangement was not without inconsistencies. Once the significance of atomic electronic configurations was realized, the table was adapted to be almost completely consistent with them. There are minor discrepancies, such as copper seeming to prefer the arrangement d10s1, rather than d9s2 as one might expect. As the long form places them, the lanthanoids and actinoids sit rather uncomfortably each in a single place, but should lanthanum and actinium be grouped directly with their congeners? Hydrogen is always a problem. In the conventional long form, hydrogen appears as in the first short period along with helium, as a kind of pseudo-alkali metal. Logically, however, one might consider it as having an outer shell lacking one electron for completion, so that it is also a kind of halogen, maybe a pseudo-halogen, though that term may confuse some readers. There is no absolutely satisfactory position for hydrogen (Scerri, 2007).
Some people have found the simple long-form periodic table unsatisfactory for aesthetic reasons. The possibility of producing a “best” form has been discussed recently (Scerri, 2008). Just as the d-transition elements were introduced into the old short form of the table to make a long form (figure 1), the actinoids and lanthanoids have been further inserted to make an even longer form (figure 2). A plethora of shapes have been proposed for the table, both two- and three- dimensional, and triangular or circular or square. For a selection see van Spronsen, 1969. None is ultimately the “correct” or “best” shape. Teachers have sometimes found the long form unsatisfactory for instructing students in detail, and various longer forms are often suggested, for example, to try to regularize the treatment of the lanthanoids and actinoids. For these and related reasons, new forms of the periodic table are being continually proposed. A “left-step” example has recently been mentioned (Scerri, 2008). This proposal is shown in figure 3. There are many others. However, it is really the person using the table who must determine if it meets his or her requirements.
|Figure 2. A long, long form of the periodic table which retains the approved 1–18 Group numbering but omits any individual group designation for the lanthanoids and actinoids.
As an inorganic chemist with experience in nomenclature, I receive many of these new versions, often with a request that IUPAC consider formally approving them. So far, our invariable response has been to refuse, regardless of the merit of the proposal. The Division of Nomenclature and Structure Representation has recently reaffirmed this position.
|Figure 3. A left-step form of the periodic table with the 1–18 Group numbering, adapted from Scerri (2008).
Some of the proposals for the table are baroque in the extreme, but that has not been the direct reason for refusing to adopt them. The proponents are often teachers who wish to use a periodic table to facilitate their own teaching of electronic structures. This is eminently reasonable, because it is not unexpected that a table based originally on atomic weights does not accord satisfactorily with every nuance of electronic structure. However, in this context, IUPAC does not concern itself with electronic structure. IUPAC’s primary concern is with unequivocal and unambiguous communication.
Older readers will remember the problem that existed before about 1985 with the different Group designations for the same group of elements, such as IVA and IVB, employed on opposite sides of the Atlantic. The reader of a paper from this time might indeed have to remember where the writer of the article was based. Some papers never contained even one element name or symbol, but would simply use phrases such as “the elements of group IVA.” Confusion was common. IUPAC needed a numbering system that was both unambiguous and comprehensive, and that would be comprehensible to chemists at every level of expertise, simple enough for beginning students and Nobel Prize winners alike. For that reason it could not be too long or too complex. A form of column numbering, 0–17, was apparently first proposed by Ölander in 1956 (Fluck, 1988), and the related 1–18 Group numbering later propagated by IUPAC seemed (and still seems) to be the most generally useful. It is now widely accepted.
|A table showing the periodicity of the properties of many chemical elements, from the first English edition of Dmitrii Mendeleev’s Principles of Chemistry (1891, translated from the Russian fifth edition).
The 1990 edition of Nomenclature of Inorganic Chemistry, page 280, described quite clearly what was and still is IUPAC’s position concerning forms of the periodic table and Group numbering: “While it is neither the intent, nor the purpose of the IUPAC Commission on the Nomenclature of Inorganic Chemistry (since 2000 superseded by the IUPAC Chemical Nomenclature and Structure Representation Division) arbitrarily to set the format of the Periodic Table to be used in all parts of the world, it is the responsibility of the Commission to offer broadly useful nomenclature proposals where direct conflicts in usage occur. After extensive discussions and many public appeals for comment, the Commission concluded that the use of the 1 to 18 numbering for the 18 columns provides a clear and unambiguous labelling for reference. . . .”
This is still (2008) the opinion of the division. For references to this discussion see Fernelius and Powell, 1982; Fluck, 1988; Fernelius, 1986; and Emsley, 1985. The long, long forms of the periodic table with 32 columns shown in figures 2 and 3 also show the 1–18 Group numbering. Continuous numbering from 1 to 32 across all 32 columns would not really meet IUPAC requirements.
Nevertheless, teachers and others should not hesitate to develop new forms of the periodic table, and to publish them if they so wish. However, they should not amend the recommended 1–18 Group numbering unless and until they can propose something that meets both their own criteria and those of IUPAC for clarity, simplicity, and brevity. Above all, they should refrain from approaching IUPAC for approval of a new form of periodic table unless these criteria are met. Until then, there is unlikely to be a definitive IUPAC-recommended form of the periodic table.
Emsley, J. (1985), New Scientist, March 7, p.33.
Fernelius, W.C., and Powell, W. H. (1982), J. Chem. Ed., 59, 504.
Fernelius, W.C. (1986), J. Chem. Ed., 63, 263.
Fluck, E. (1988), Pure Appl. Chem., 60, 431.
Holden, E. N., and Coplen, T. (2004), Chemistry International, 26(1).
Lavelle, L. 2008, J. Chem. Ed., 85, 1482.
Paneth, F. 1923, Z. Angew. Chem., 36, 407.
Scerri, E.R. (2007), Chemical Heritage Magazine, 25(1), http://www.chemheritage.org/pubs/ch-v25n1-articles/
feature_mendeleev.html, accessed 30 September 2008.
Scerri, E.R. (2008), J. Chem. Ed., 85, 585; Collected Papers on the Philosophy of Chemistry, Imperial College Press, p. 179; International Journal of Quantum Chemistry, 109, in press.
Van Spronsen, J.W., 1969, The Periodic System of the Chemical Elements, Elsevier, Amsterdam.
Jeffery Leigh <[email protected]> is a member of the Chemical Nomenclature and Structure Representation Division (IUPAC Division VIII). He is an emeritus professor of Environmental Science at the University of Sussex.
last modified 6 January 2009.
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